The present invention relates to an improved, low cost, fluid flow assisted ion projection printing head which has surfaces effected by extended exposure to the chemistry of the corona discharge coated with an aluminum hydroxide film.
One approach to providing a reliable high resolution on contact printing system is the fluid assisted ion projection printing which in one form entails depositing electrostatic charges in a latent image pattern directly upon a charge receptor surface and then rendering the charge pattern visible in some known manner. Typically, ion protection printing comprises the generation of ions in an ion stream and the control of the ions which may reach a charge receiving surface.
In two patents assigned to the same assignee as the instant application, there are disclosed different forms of a fluid jet assisted ion projection printing apparatus. In each of U.S. Pat. No. 4,463,363 entitled "Fluid Jet Assisted Ion Projection Printing" (Robert W. Gundlach and Richard L. Bergen) and U.S. Pat. No. 4,524,371 entitled "Modulation Structure for Fluid Jet Assisted Ion Projection Printing Apparatus" ((Nicholas K. Sheridon and Michael A. Berkovitz), there is disclosed an ion generation chamber through which air is moved for entraining ions generated therein and for transporting them through an exit channel including an ion modulation region for subsequent deposition upon a latent image receptor. In U.S. Pat. No. 4,463,363, the entire exit channel, including the modulation region, forms a straight path extending from the ion generation chamber to the image receptor. In U.S. Pat. No. 4,524,371, the improvement over the U.S. Pat. No. 4,463,363 structure resides in the exit channel defining a bent path through which the ions flow, in order to allow the ion modulation control elements to be fabricated upon a planar substrate.
In both of these patents, the ion generation chamber is formed as a substantially cylindrical cavity within which the corona wire is centrally located. It was believed that the cylindrical configuration was necessary in order to obtain a stable corona discharge from the corona wire. The high electrical fields established between the axially mounted corona wire, maintained at several thousands volts d.c., and the equidistant conductive walls of the cavity, were expected to cause arcing to any portion of the cavity walls which were non-smooth or to any corners therein where electrical lines of force would be concentrated.
In a third patent, also assigned to the same assignee as the instant application, U.S. Pat. No. 4,644,373, entitled "Fluid Assisted Ion Projection Printing Head" (Nicholas K. Sheridan, Gerhard K. Sander) an improved ion projection printing head is described. In this embodiment the printing head comprises a one piece conductive body which may be easily cast and which mates with a substantially flat conductive plate against which a second planar member supporting electronic control elements is held. The corona wire is located closer to the conductive wall and the conductive plate than to any of the other walls of the ion generating chamber for concentrating the major portion of the electrical field between the wire and these elements as opposed to any other portions of the chamber walls when the wire is connected to a source of electrical potential.
Typically, these printing heads are made by casting electrically conductive material such as stainless steel which although satisfactory in most respects is also very expensive compared to other conductive materials that are available. The use of die cast aluminum, for example, in place of die cast stainless steel would reduce the expense of the printing head by more than 50%, if not 75%. Aluminum, however, suffers from the problem that it is effected by extended exposure to the chemistry of the corona discharge in that it oxidizes after a relatively short period of time of from about 4 to 10 hours forming an insulating aluminum oxide film which builds up as a preceptable film on the printing head thereby reducing the total output ion current to an unacceptable level. The problems with aluminum are further compounded by the fact that ammonia is a common low level pollutant in most atmospheres and that in the presence of corona discharge the oxygen and nitrogen in the air combine with the ammonia to form ammonium nitrate which deposits on the printing head as a visible white powder or whisker. While similar difficulties exist with regard to the formation of the ammonium and potassium nitrate on a stainless steel head, they are not to the degree found with the use of an aluminum head because the difficulty is exacerbated by the formation of an aluminum oxide film on the head.
In an attempt to avoid these difficulties, various proposals have been made for the use of ammonia filters to rid the atmosphere of ammonia which would otherwise combine with the oxygen and nitrogen under the influence of the corona discharge. In addition gold has been used to provide an inert conductive film on the printing head. Such a film, however, like the stainless steel, is highly susceptible to the formation of a ammonium nitrate deposits which reduce the ion current to an acceptably low level.